Photopolymerization is a process which leads to the formation of a polymer from monomeric molecules. Many photopolymerizable materials are known in the art, and have been used to form resist and other types of images.
A good review of photopolymerization processes appears in Chapter X of Imaging Systems, Jacobson and Jacobson, John Wiley & Sons, N.Y., N.Y., pp. 181-222 (1976). Dye-sensitized photopolymerization systems are discussed in some detail; see, for example, pp. 184, 195-197, and 214-216. A wide variety of ethylenic unsaturated monomers are noted as useful in such systems. The use of methylene blue as a dye sensitizer in combination with sodium p-toluene sulfinate as a polymerization initiator is noted as a particularly useful photopolymerization initiation system with barium diacrylate and acrylamide, and such a photopolymerizable composition also is noted as useful as a holographic recording material. U.S. Pat. No. 3,694,218 issued to Margerum et al on Sept. 26, 1972 describes the formation of holograms using a similar photopolymerization system comprising, e.g., barium diacrylate, lead diacrylate, acrylamide, methylene blue, p-toluenesulfinic acid sodium salt and 4-nitrophenylacetic acid sodium salt.
Other polymerization initiators disclosed in said U.S. Pat. No. 3,694,218 include triorgano phosphines, triorgano arsines and other organic sulfinic compounds. Similar disclosures of polymerization initiators appear in U.S. Pat. No. 3,573,922 issued Apr. 6, 1971 and U.S. Pat. No 3,649,495 issued Mar. 14, 1972, both in the name of Rust.
Photopolymerization systems of the type described in the aforementioned Margerum and Rust patents are liquids, and are used by being placed between glass slides spaced apart to form a thin cell. The addition of a film-forming thickener to increase viscosity or to form a film has been proposed in U.S. Pat. No. 3,594,204 issued Aug. 3, 1971 to Rust; examples of proposed polymeric thickeners include gelatin, polyvinyl alcohol, polyvinylpyrrolidone and cellulosic compounds. A similar disclosure appears in U.S. Pat. No. 3,531,281 issued Sept. 29, 1970 to Rust, wherein the addition of polyvinyl pyrrolidone is said to be for the purpose of adding viscosity to the solution so that it would form a more satisfactory film on glass. U.S. Pat. No. 3,658,526 issued Apr. 25, 1972 to Haugh discloses the addition of a variety of polymers to form photopolymerizable films using monomers having a boiling point above 100.degree. C.
The formation of phase holograms by photopolymerization using barium diacrylate in combination with methylene blue and p-toluene sulfinic acid is discussed at some length by Jenney in an article "Holographic Recording with Photopolymers", Journal Optical Society of America, Vol. 60, pp. 1155-1161, September 1970.
Margerum et al have reported on a number of "catalysts" or polymerization initiators used in combination with methylene blue and acrylamide in a paper "Studies on the Mechanism of Dye-sensitized Photopolymerization Initiation", Polymer Preprints for the 160th American Chemical Society Meeting, Chicago, Ill. Sept. 15, 1970, pp. 634-644. In some instances, a mixture of acrylamide and barium diacrylate was used. Margerum et al concluded that the sulfinate ions were far superior to such other catalysts as triethanolamine and ethylenediamine tetraacetic acid (EDTA). Sugawara et al (Applied Optics, Vol. 14, February, 1975; pp. 378-382) have reported on the use of acetylacetone and triethanolamine as initiators with acrylamide and methylene blue.
It has been recognized that such photopolymerization systems generally have limited shelf lives--sometimes only a few hours--even though the monomers and photocatalyst systems individually have long shelf lives; note Jenney, supra, p. 1155 and Sugawara et al, supra, pp. 378 and 382. For this reason, the art has taught that the components should be mixed and coated just prior to use. Indeed, U.S. Pat. No. 4,036,647 issued July 19, 1977 to Brault et al proposes a structure in which part of photopolymerizable composition is coated on a support while the remaining components are contained in a liquid stored in a rupturable container or pod. Rupture of the pod and distribution of the liquid between said coated support and a second element forms the photopolymerizable composition in situ immediately prior to use.
U.S. Pat. No. 4,173,474 issued Nov. 6, 1979 in the name of Tanaka et al. discloses the formation of holograms by the photopolymerization of a monomer, e.g., barium acrylate, in a carrier polymer, e.g., gelatin, followed by immersion in one or more solvents such as isopropanol. Tanaka et al mention polyethylenimine and polyvinyl pyrrolidone in a long list of alternative polymers suitable for use as the carrier polymer; note col. 3, line 67 to col. 4, line 19. The carrier polymer is selected on the basis of its swellability when contacted by a solvent after photopolymerization; see col. 2, lines 23-25. There is no white light exposure to fix the laser exposed coating. The patent does not disclose or suggest applicants' discovery that branched polyethylenimine is an active participant in dye sensitized photopolymerization of ethylenic unsaturated monomers. Moreover, Tanaka et al neither disclose nor suggest the use or special properties of lithium acrylate as the photopolymerizable monomer.